The invention relates to a metallurgical container comprising at least one hearth bottom electrode, for example a steel plant arc furnace operated with direct current.
Such a container comprises, generally speaking, a metallic vessel internally lined with refractory material and at least a device for the current supply, usually called "hearth bottom electrode", which passes through the bottom of the container in order to provide the feedthrough of electric current between a source of electric energy and the charge of metallic material to be molten and/or to be heated, which is in the interior of the container.
In the case of a direct current steel plant arc furnace, the container comprises also at least one roof electrode connected to the source of electric current. The electrode is put in place above the charge and is movable vertically, so as to create between the charge and the roof electrode an electric arc.
Several types of hearth bottom electrodes are known, particularly an electrode having a cylindrical general shape and comprising a metallic bar, whose upper end is flush with the surface of the refractory lining and whose other end is linked to a connector constituted by a conductive material which projects below the vessel and is connected to a source of current. Such an electrode is described in the document FR-A-2577311. The upper part of the bar, which partly melts during operation, is generally made out of steel in the case of a steel plant furnace, whereas the connector with its inner cooled parts is formed of copper or of a copper alloy. According to a particular embodiment, described in the document EP-A-472254, the interface between these two parts is situated at a level located within the thickness of the refractory lining, sufficiently high to avoid as far as possible the heating and the partial melting of the bar. In view of the bearing of this electrode, it is foreseen that the upper part should have a diameter which is greater than that one of the lower part in order to form a shoulder which is resting on a ring surrounding the lower part and is rigidly tied up to the vessel.
It is moreover known that during the use of these furnaces, and as a result of the melting and resolidification cycles of the upper part of the hearth bottom electrode, this latter undergoes during each cycle a slight displacement along its axial direction. The displacements accumulated during the cycles are responsible for a rising of the electrode towards the inside of the furnace. In order to avoid this rising of the electrode it has been considered, in assemblings which are of a type different from that one described above, to provide the hearth bottom electrode with return movement means intended to oppose these displacements.
Thus, in the already cited document FR-A-2577311, a hearth bottom electrode has been described which is constituted by a steel bar with a constant diameter whose lower end is surrounded by a cooled jacket. In this case the lower end of the bar is mechanically tied up to the vessel of the furnace with the help of supporting means, which latter also comprise means for the elastic return movement of the electrode towards a lower level.
The application of such means to the hearth bottom electrode of the type described in the first place, however, does not allow to effectively and durably prevent its rising, due to the fact that materials can penetrate, for example by infiltration, between the shoulder of the bar and the ring, and prevent the return movement of the electrode towards the lower level.
During the course of a campaign of operation of the furnace, the accumulated displacements of the electrode towards a higher level may result not only in a reduction of the height of the bar due to a consumption of the electrode starting from the upper side, but also in a bringing the lower cooled connector dangerously close to the front of the heat.